Audio signals received from audio communication networks often contain signals that when converted to sound may cause physical damage to a listener's ears or cause other symptoms, such as acoustic shock. The prior art comprises several disclosures of means for preventing such undesired or potentially harmful content from reaching the ears of the listeners.
In the article, “Subband-Based Acoustic Shock Limiting Algorithm On A Low-Resource DSP System” from “Proc. EUROSPEECH”, 2003, pp. 2869-2871, Choy et al. disclose an algorithm that processes the input signal in both the time and frequency domains. This approach allows the algorithm to detect sudden increases in sound level (time-domain), as well as frequency-selectively suppressing shock disturbances in the frequency domain. The unaffected portion of the sound spectrum is thus preserved as much as possible. A simple algorithm calibration procedure is proposed to satisfy different sound pressure level (SPL) limit requirements for different communication equipment. The implemented algorithm limits narrowband SPL at different frequencies to predefined limits. Limiting is accomplished by computing subband gains in dependence on subband levels. When a shock is detected in a subband, the corresponding subband level and gain are computed with a shorter averaging time; when no shock is detected in a subband, the corresponding subband level and gain are computed with a longer averaging time. The authors further propose adding broadband shock disturbance limiting ability, e.g. by making the gain calculation adaptive to the flatness of the signal spectrum. Finally, the authors suggest a possible approach to incorporating detection and virtually elimination of periodic acoustic shock disturbance in each subband.
U.S. Patent Application 2005/0018862 discloses a method and an apparatus that may control the exposure of a listener to narrow-band signals in an audio signal. An amplifying device detects the presence of one or more high-pitched narrow-bandwidth signals within audio telephony signals, in isolation or in the presence of speech signals, and performs rapid, selective attenuation of the one or more narrow-bandwidth signals. A narrow-band signal may be detected based on whether the ratio of the signal level in a particular frequency region to the signal level in nearby higher and lower frequency regions exceeds a predetermined threshold. Attenuation is made by band-reject filters having centre frequencies that approximately match the frequencies closest to the frequencies of the shrieks detected. The degree of attenuation provided at the frequency of the shriek may be progressively increased as the duration of the shriek increases.
U.S. Pat. No. 7,565,283 B2 discloses a method of controlling potentially harmful signals in a signal arranged to convey speech, the method including the steps of: analysing the signal to determine features of the signal; analysing the signal to determine if it is conveying a speech signal; if the signal is determined to be conveying a speech signal then establishing characteristics of the signal based on the analysed features of the signal; and if a feature of the signal exceeds a corresponding established characteristic then controlling the signal to reduce the excess. The features may include any measurable feature including the amplitude of the signal, the power of the signal, the loudness of the signal, the amplitude of the signal in particular frequency bands, the power of the signal in particular frequency bands, and the specific loudness of the signal; all of the aforementioned being measured on a time dependent basis. By this method, the signal is controlled relative to the characteristics previously determined from the signal when it was conveying speech. If the signal exhibits features outside the established characteristics then it is controlled. Hence, reduction in the amplitude of potentially harmful signals may be provided. Further, control of potentially harmful signals may be provided independent of the sensitivity of the specific sound reproduction equipment employed by the user.
While the above cited disclosures provide protection against several types of undesired or potentially harmful content and at the same time a significant degree of preservation of desired signal content, there is still a need for improvement.